Anodic phosphating of metallic articles

ABSTRACT

A METALLIC ARTICLE IS IMMERSED IN SODIUM PHOSPHATE SOLUTION AS THE POSITIVE ELECTRODE AND, MAINTAINING THE POTENTIAL IN THE RANGE OF -0.4 V. TO +0.2 V., A PH OF 3-5.5, AND THE CELL CURRENT IN THE RANGE OF 0.9 A/SQ. FT. TO 22.5 A./SQ. FT., A HEAVY COATING IS PRODUCED.

United States Patent 015cc 3,586,612 Patented June 22, 1971 ANODIC PHOSPHATING OF METALLIC ARTICLES Kummattithidal S. Rajagopalan, Balasubramanya Dandapani, Anathakrishnan Jeyaraman, and Chakravarti Rajagopal, Karaikudi, India, assignors to Council of Scientific and Industrial Research, New Delhi, India No Drawing. Filed Mar. 27, 1969, Ser. No. 811,243

Int. Cl. C23b 11/00 US. Cl. 204-56R 9 Claims ABSTRACT OF THE DISCLOSURE A metallic article is immersed in sodium phosphate solution as the positivev electrode and, maintaining the potential in the range of 0.4 v. to +0.2 v., a pH of 3-5.5, and the cell current in the range of 0.9 a./sq. ft. to 22.5 a./ sq. ft., a heavy coating is produced.

This invention relates to the anodic phosphating of metallic articles.

It is well known that phosphate coatings impart valuable properties to metallic articles; for instance, (a) they improve the adhesion of paints on metals; (b) they retard the spread of corrosion under the paint film from those places where the paint has been damaged; (c) they lengthen the life of paint coatings; (d) they reduce friction in deep-drawing operations such as wire making, tube making and body-pressing; and (e) they reduce wear and galling on mating surfaces.

Phosphate coatings are made on articles of iron, steel, zinc, cadmium and various other metals.

The said coatings are produced by treating the metallic articles with phosphoric acid or with solutions of primary phosphates of manganese, iron, zinc or cadmium. The coatings produced at present vary between 100 mg.,- to 3000 mg., per square foot.

The conventional methods employed for producing phosphate coatings suffer from various drawbacks, such as: (i) that for obtaining heavy coatings the phosphating has to be done at temperatures ranging from 85 C. to 100 C.; (ii) that heavy metal phosphates have to be employed as principal ingredients of the bath compositions; (iii) that there is need for the ageing of the baths; (iv) that there is need for frequent checking up of the bath composition; and (v) that the surface condition of the metal has a considerable influence on the quality of the coating produced.

Attempts hitherto made to produce phosphatic coatings from sodium phosphate solution containing accelerators have resulted only in the production of thin coatings having coating less than 500 milligrams weight per square foot.

This invention has for its objects an improved process of phosphating metal articles, which process will not only be free from the drawbacks above-mentioned, but'will also enable the production of heavier coatings up to about 6000 mg., weight per square foot.

This invention is based on the discovery that these objects can be achieved by (a) using only sodium phosphate solution, as the treating solution, (b) by adjusting the concentration of the sodium phosphate solution, by adjusting the pH of the said solution, and (d) by adjusting the cell current so as to maintain the electrode potential at a predetermined range.

Based on this discovery, this invention broadly consists of an electrochemical process for phosphating a metallicarticle wherein the metallic article to be phosphated is after polishing and degreasing immersed in a 2.5 %-5 sodium phosphate solution which is maintained at a pH value in the range of pH 3 to pH 5.5, the

said article being made the positive electrode (anode), the potention of the metallic article being maintained in the range of 0.4 v. to +0.2 v. (vs. S.H.E.) and the cell current being maintained in the range of 0.9 a./sq. ft. to 22.5 a./sq. ft.

The aforesaid process may be carried under conditions set forthbelow:

(i) The potential of the anode may be maintained at the appropriate value by passing an appropriate current through the cell, or the electrode--solution potential value may be controlled and retained at any pre-determined value by employing a potentiostat for controlling the potential.

(ii) The pH of the bath may be adjusted either by addition of phosphoric acid or by passing the sodium phosphate solution through an ion exchange column.

(iii) The process may be carried out at room temperature of about 20 to 40 C.

(iv) The negative electrode or cathode may be made of a steel mesh or plate or the tank itself can be made the cathode.

(v) A stabilised DC. voltage source should be employed.

(vi) The weight of the phosphate coating may be varied by adjusting the anode potential or the cell current, the pH of the solution and concentration of sodium phosphate.

(vii) The electrolytic tank may be made of steel, glass, wood or any material which is not affected by sodium phosphate solution.

(viii) The said anodic process can be carried out from bath compositions in the formation of which no heavy metal phosphate is employed.

(ix) Frequent check-up of the bath composition, as in chemical phosphating is not necessary in the anodic phosphating according to this invention.

(x) Ageing of the bath,.as in chemical phosphating, is not necessary for carrying out the anodic phosphating.

This invention will now be more particularly described with reference to a few examples.

EXAMPLE 1 Phosphate coating as a base for painting .of the potentiostatadjust the potentiometer setting of the potentiostat to l.25 v. vs. S.H.E. for 1 minute and change it to -0.25 v. vs. S.H.E.

The current measured will be seen to increase and fall to zero. Now, switch ofi and take out specimens, wash and dry. The specimen is ready for painting. The coating weight will be about 500 mgms./sq. ft. The coated specimen will have a suitable base for painting.

EXAMPLE 2 Phosphate coating as a protective finish Prepare 2.5% disodium hydrogen phosphate, adjust to pH 4 and proceed as before. Change the potentiometer setting from 1.25 v. vs. S.H.E. to +0.20 v. vs. S.H.E. As soon as the current becomes zero, switch otf the potentiostat, take out the specimen, wash and dry. The coating weight will be 1.4 gms./sq. ft., and the phosphated specimen is ready for sealing with oil or grease.

3 EXAMPLE 3 Phosphate coating to reduce wear and friction Prepare 2.5% disodium hydrogen phosphate, adjust to pH 4 and proceed as before. Change the potentiometer setting from 1.25 v. vs. S.H.E. to 0.2 v. vs. S.H.E. As soon as the current comes to zero, switch oif, remove the specimen and dry. As soon as the current comes to zero, switch off, remove the specimen wash and dry. Now the coating weight will be almost 6 gms./sq. ft. and the specimen is ready to be handled for deep drawing operations.

EXAMPLE 4 Phosphate coating on other metals Prepare 2.5% disodium hydrogen phosphate, adjust to pH 4 and proceed as before but use as zinc specimen in place of steel specimen. Change to potentiometer setting from -1.25 v. vs. S.H.E. to 0.4 vs. S.H.E. As soon as the current comes to zero, switch oif the potentiostat, remove the specimen, wash and dry. The coating weight will be 1.35 gms./sq. ft., and the specimen is ready for painting.

EXAMPLE 5 Effect of noble potential on weight of coating The following table gives the experimental data of certain experiments carried out with a view to ascertain whether the value of the noble potential will have any effect on the weight of the coating deposited, other factors remaining the same; these experiments were carried out at room temperature (30 C.), using a bath composition having 1.7% P and 0.8% Na:

Potential vs. Coating weight in S.H.E., v.: gms./sq ft EXAMPLE 6 Effect of pH of bath composition on coating weight The following table gives the experimental data of certain experiments carried out with a View to ascertain how the coating weight will be affected by pH value of the bath composition; in these experiments-- P0 and Navalues were the same as in Example 4; the temperature 'was the room temperature; and the value of the noble potential above the equilibrium potential was 0.25 v. vs.

EXAMPLE 7 Effect of bath composition conditions on appearance of coating The following table gives the result of certain experiments carried out at constant potential with a view to ascertain how the appearance of the coating is aifected by the variation of the experimental conditions; the experiments were carried out at room temperature of 30 C.

Bath composition P04 Na pH Appearance of coating 1. 7 0. 8 4. 5 White crystalline film. 1. 7 0. 8 4. 0 Black crystalline film. 3. 4 1. 6 4. 0 Crystalline white film.

4 EXAMPLE 8 Effect of variation of potential on appearance of coating The following table gives the results of certain experiments carried out at a constant pH of 4, with a view to ascertain how the appearance of the deposit is affected by the variation of the potential; the experiments were carried out at room temperature (30 C.) using a bath of composition having PO 1.7% and Na 0.8%:

The following table gives the result of certain experiments carried out by passing a constant current from a rectifier.

Bath composition Current density, Cell Coating P04 Na amp/sq. it. voltage weight What we claim is:

1. An electrochemical process for phosphating a metallic article wherein the metallic article to be phosphated is, after polishing and degreasing, immersed in a solution consisitng essentially of 2.5 %5% aqueous sodium phosphate-phosphoric acid solution which is maintained at a pH value in the range of pH 3 to pH 5.5, the said article being made the positive electrode (anode), the potential of the metallic article being maintained in the range of 0.4 v. to +0.2 v. (vs. S.H.E.) and the cell current being maintained in the range of 0.9 a./sq. ft. to 22.5 a./sq. ft.

2. An electrochemical process as claimed in claim 1, wherein the potential of the anode is maintained at the appropriate. value by passing an appropriate current through the cell.

3. An electrochemical process as claimed in claim 1, wherein the electrode solution potential value is controlled and retained within said range by employing a potentiostat for controlling the potential.

4. An electrochemical process as claimed in claim 1, wherein the pH. of the bath is adjusted by passing the sodium phosphate solution through an ion exchange an ion exchange column.

5. An electrochemical process as claimed in claim 1, wherein the process is carried out at a temperature of about 20 C. to 40 C.

6. An electrochemical process as claimed in claim 1, wherein the negative electrode or cathode is made of steel mesh, steel plate or the tank itself.

7. An electrochemical process as claimed in claim 1, wherein a stabilised DC. voltage source is employed.

8. An electrochemical process as claimed in claim 1. wherein the weight of the phosphate coating is varied by adjusting the anode potential and the cell current.

9. An electrochemical process as claimed in claim 1, wherein the electrolytic tank provided for the metallic article is made for a material which is not adversely atfected by sodium phosphate solution.

References Cited UNITED STATES PATENTS 870,937 11/1907 Coslett 204-56 FOREIGN PATENTS 486,752 6/1938 Great Britain 204--56 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner 

